Multicast Pre-Distribution in VoD Services

Noriaki Kamiyama, Ryoichi Kawahara,

Tatsuya Mori, Haruhisa Hasegawa

NTT Service Integration Laboratories

2011. 5. 11

Problem in VoD System

Demand in VoD concentrates at night and changes widely on daily basis.

2

Need to design VoD system based on demand at peak hours to maintain stable service quality

Important for ISPs to reduce server and network load at peak hours to suppress network cost

12/1Date

0

5

10

15

20

25

Max

imum

act

ive

sess

ion

coun

t in

each

hou

r [1

02 ]

2 3 4 5 6 7

Max. active session count in each hour for seven days from Dec. 1, 2004 in VoD service of China Telecom

Demand at peak hours is five times larger than that at off-peak hours.

Multicast Pre-Distribution (MPD) VoD

Users are provided huge amount of access link bandwidth much larger than playback rate.

Large part of downlink bandwidth remains unused.

Effective to always deliver content and store them at STBs independently of user requests

Video server can freely select delivery content and freely set time at which each content is delivered.

Delivers some popular content to all users in multicast session

Propose MPD VoD system

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Overview of MPD VoD Video server always pre-distributes popular content to

all users, and STBs store all of them. Determines pre-distribution schedule on daily basis Users can watch pre-distributed content without

generating load at server and network at request. Delivers content on-demand in unicast session for

requests of content not pre-distributed

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Server

NetworkSTB

TV

Multicast pre-distribution

On-demand unicast delivery

Watching pre-distributed content

Assumptions

Set transmission rate of each MPD channel as

Rb = Ad – R (Ad: downlink capacity, R: playback rate)

Video server makes pre-distribution schedule for 24 hours in n-th day at beginning of n-th day (0:00)

To preserve copyright of content, remove pre-distributed content from STBs at 0:00 after L days elapse

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Selecting Pre-distributed Content

To improve reduction effect of server and network load in n-th day, desirable to pre-distribute content m with large Dm,n, total download time (TDLT) for content m in n-th day

Estimates Dm,n using EMA (exponential moving average):

Dm,n = Dm,n-1 + (1 - )Dm,n-1 (: smoothing parameter)

Except content pre-distributed between (n-L)-th day and (n-1)-th day, selects content in descending order of Dm,n

Pre-distributes selected content in descending order of estimated request count Dm,n/Sm (Sm: length of content m)

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Access Log Data Used in Evaluation

Access log data in PowerInfo VoD system, commercial VoD service of China Telecom

Seven months from 2004/6/1 to 2004/12/31 Request count: 20,921,657, content count: 6,735 User count: 37,360, average content size: 3,510 seconds

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10-1

1

1 10 102

CC

D

Content size (minutes)

10-2

10-3

10-4

Average TDLT per day (hours)

CC

D

Users Content10-1

1

10-2

10-3

10-4

10-5

103 1 10 10210-5 10-4 10-3 10-2 10-1 103

40% content was 45 minutes. Another 40% content was 90 minutes.

Few users generated many requests. Many requests concentrated on specific content.

Evaluation Condition

Transmission capacity of downlink: Ad = 10 Mbps Playback rate: R = 2 Mbps Transmission rate of MPD channel:

Rb = Ad – R = 8 Mbps Average content count pre-distributed on each day:

X = 98.46 Smoothing parameter of EMA: = 2/3

(Data of most recent two days occupied 86% of information used in estimation.)

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Effect of MPD on Server Load

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0

5

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20

25

Date

L = 0

12/1Date

0

5

10

15

20

25

6/1 7/1 8/1 9/1 10/1 11/1 12/1 1/1

L = 1

L = 7

2 3 4 5 6 7

L = 0L = 1

L = 7Max

imum

act

ive

sess

ion

coun

t in

eac

h da

y [1

02 ]

Max

imum

act

ive

sess

ion

coun

t in

eac

h ho

ur [1

02 ] Max. session count was largely reduced by MPD

compared with only unicast delivery (L = 0). Effect of MPD was stronger when demand was larger.

Effect of MPD on Network Load

Average link load was reduced by MPD in all 31 networks. The effect of MPD was larger as L increased.

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L: content life length in STBs [days]

0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7

Server load

Used 31 topologies of commercial ISPs

Allocated server at node giving minimum av. hop length of unicast flows

Transmitted packets on minimum spanning tree

(Av. traffic on each link using MPD) / (That using only unicast delivery)

Required Storage Size of STBs

As L increased, Kmax and V decreased.

However, as L increased, Zmax linearly increased. Just small part of pre-distributed content were viewed.

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L Kmax V(107) (%) Zmax (TB)

0 2409 8.79 *** ***

1 1738 6.34 0.231 0.69

2 1488 5.26 0.331 1.38

3 1312 4.62 0.389 2.07

4 1174 4.15 0.432 2.76

5 1059 3.77 0.465 3.46

6 972 3.45 0.492 4.15

7 916 3.18 0.514 4.84

Kmax: max. active session count over entire log period

V: average total session length in each day

: utilization of

pre-distributed content Zmax: max. required storage

size of STBs

Conclusion

Important to reduce peak load in VoD service

Proposed to pre-distribute popular content to all users on multicast session independently of actual user requests, in addition to on-demand unicast delivery

Numerical evaluation using access log of actual VoD: Reduced server and network load Reduction effect of peak server load:

30% (L = 1), 60% (L = 7) Needed several TB storage at STBs

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Merit of MPD VoD

Reduction of load of video server and network Pre-distributes popular content

Reduces number of on-demand deliveries Multicast pre-distribution

Suppresses load cause by pre-distribution

On-demand service & simple VCR operations Users view pre-distributed content from STBs. On-demand delivery for content not pre-distributed

No need for user cooperation Only user owning STB can view pre-distributed content

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VoD Service

Increase of transmission capacity of access links

Dramatically increasing user count of VoD services

VoD delivering UGC (user generated content): Provided by service providers independent of ISPs

VoD delivering rich content: Provided mainly by ISPs regarding it as important service

Server

STB(set-top box)TV

Request

Content delivery

Network

CDN

Provides multiple cache servers over network and distributes copies of content on cache servers

Delivers content from cache server close to requesting user

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Total cost of server facilities is not reduced because total load over all server facilities is constant.

Original server

Network

Cache serverSTB

TV

Multicast Delivery System

Delvers content to multiple users requesting same content on single multicast delivery session

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Increases user response time to increase user count accommodated in each multicast session

Needs complex operations to switch delivery session to support VCR operations

ServerNetwork

STB

TV

P2P Delivery System

Each peer, i.e., user, uploads viewing content to other peers viewing same content

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Needs to sustain content delivery when uploading user leaves or making VCR operations Needs complex operations, i.e., switching source peers

Needs incentive for users

ServerNetwork

STB

TV

サーバ負荷低減法 (4): ISP制御型 P2P配信

HDレコーダーの価格低下は目覚ましく，現在， 1Tバイトの容量のHDレコーダーを4万円程度で購入可能

ユーザ宅内の STBを ISPの管理下で大容量のキャッシュとして運用 ユーザ離脱への対応が不要で全体最適化が可能

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ユーザに対して ISP管理下の STBを設置させ，他のユーザへの配信に要する電力を負担するインセンティブの付与が必要

配信サーバの負荷は低減されるが， NWの負荷は低減せず

ServerNetwork

STB

TV

総視聴時間上位コンテンツの一致度

d 日間離れた日の各総視聴時間上位 x 個のコンテンツの中で，両日ともに上位 x 以内となったコンテンツの割合

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0

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1 10Av.

dup

licat

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ratio

of t

op x

co

nten

t bet

wee

n la

g d

days

x102 103 104

d = 1d = 2d = 3

d = 4

d = 5

d = 6

d = 7

x が小さい領域では，間隔 d が増加するに伴い総視聴時間上位x 個のコンテンツの一致度は低下するが， x の増加に伴い一致度は増加し， x が 1,000程度以上になると d による一致度の差異は消滅

BC事前配信コンテンツの選択効率 各日にx 個を BC事前配信した際の選択効率 : (STB上の Lx個の総視聴時間 ) / (実際の上位 Lx個の総視聴時間 )

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1 10

Ave

rage

effi

cien

cy o

f se

lect

ing

top

x co

nten

t

x

L = 1

102 103 104

L = 3L = 7

xが小さい場合， Lの増加に伴い， BC事前配信時点と各日の総視聴時間上位コンテンツの一致度が低下し，選択効率は低下

しかし選択効率の低下度合いは小さく， Lが 7日でも 70%の選択効率

xの増加に伴い選択効率は向上し， x 10であれば 85～ 90%を達成

⇒ EMAを用いた総視聴時間の推定値に基づき BC事前配信コンテンツを選択することで，十分な選択効率が達成可能